a. Field of the Invention
This invention relates to a drum brake. In particular, the invention relates to an improved brake shoe for a drum brake that enables more consistent brake dynamics, control and performance as compared to conventional brakes.
b. Background Art
In a conventional drum brake, a brake drum rotates with a wheel or wheels proximate to one end of an axle. The drum defines a radially inner braking surface. A brake spider is disposed about the axle and a pair of brake shoes are pivotally mounted at one end to the brake spider. The opposite end of each brake shoe is engaged by an actuating member such as a cam or hydraulic piston or wedge to move the brake shoes between positions of engagement and disengagement with the braking surface of the brake drum.
Although drum brakes are generally less expensive than disc brakes, drum brakes may suffer from inconsistent brake dynamics, control and performance as compared to conventional disc brakes. For example, drum brakes have a self-energizing characteristic. During braking, rotation of the brake drum imposes a force urging the leading brake shoe into engagement with the braking surface of the brake drum (while urging the trailing brake shoe out of engagement with the braking surface of the brake drum). This characteristic often results in application of a braking force greater than desired at low speeds, but less than desired at high speeds and is particularly pronounced when using friction materials with a relatively high coefficient of friction. Further, this characteristic results in uneven braking forces applied by the leading and trailing brake shoes with the difference in braking force applied by the shoes increasing as the coefficient of friction increases. This characteristic also results in uneven wear on the brake shoes. Conventional drum brakes also require the use of relatively large anchor pins to pivotally couple the brake shoes to the brake spider. Because the pins are relatively large, the radial distance from the center of the brake to the pivot axis of the anchor pin is relatively small. The relatively small radial distance increases the self-energizing characteristic of the brake and results in even greater variance in the braking force between leading and trailing brake shoes.
Conventional brakes shoes for drum brakes also may have difficulty in handling certain loads in view of their construction. A conventional brake shoe includes a brake table that supports the brake lining and multiple webs that support the brake table and couple the brake shoe to the anchor pin and actuating member at opposite ends. As a result of this configuration, the webs must both support the brake table and carry loads from the anchor pins and actuating member. The webs therefore are made from relatively heavy materials and must be machined to provide appropriate engagement surfaces.
The inventor herein has recognized a need for a brake shoe that will minimize and/or eliminate one or more of the above-identified deficiencies.